1. Field of the Invention
The present invention relates to a separator for a fuel cell in contact with a pair of electrodes interposing an electrolyte film and a fuel cell using the aforementioned separator.
2. Description of the Related Art
A fuel cell is known as an apparatus for converting fuel energy directly to electric energy. The fuel cell is generally designed to be provided with a pair of electrodes with an electrolyte film interposed therebetween and to generate energy from the space between the pair of electrodes by an electrochemical reaction of fuel gas, e.g. hydrogen, and oxygen-containing gas. In this reaction, fuel gas is supplied to contact the surface of one of the electrodes and oxygen-containing gas is supplied to contact the surface of another electrode. Energy can be drawn from the fuel cell in a highly efficient manner as long as fuel gas and oxygen-containing gas are supplied.
FIG. 41 is a perspective view showing the configuration of a stack structure 5 constituting a general fuel cell and FIG. 42 is an exploded perspective view showing the structure of a unit cell 10 as a basic unit of the stack structure 5 shown in FIG. 41. In general, the fuel cell, for example, of a polymer electrolyte type is constituted of the stack structure 5 as shown in FIG. 41. This stack structure 5 is produced by laminating a prescribed number of unit cells 10, then disposing collector plates 26, 27, insulating plates 28, 29 and end plates 40, 45 sequentially at both ends of the unit cells and then fastening these ends using, for example, bolts and nuts such that it is maintained in the state where a given pressure is applied in the direction (the direction indicated by the arrow) of the lamination of the unit cell. The collector plates 26, 27 are provided with output terminals 26A, 27A respectively which enable it to output the electromotive force generated in the fuel cell structured by the stack structure 5.
In such a fuel cell, a member called a separator is provided which serves as a gas passage and a collector electrode to supply fuel gas and oxygen-containing gas to the electrode surface. A straight type separator provided with a plurality of linear passage grooves has been conventionally used. Serpentine type separator in which one passage groove is bent (disclosed in Japanese Patent Application Laid-Open (JP-A) No. HEI 7-263003) and lattice type separators in which plural projections are arranged and a passage is formed by a gap between these projections have also been known.
The unit cell 10 as a basic unit of the stack structure 5 of FIG. 41, as shown in FIG. 42, includes a joint body (reaction electrode layer) 15 produced by sandwiching an electrolyte film 11 between a cathode 12 and an anode (not shown), and separators 20A, 20B (the lattice type is shown as example) disposed on both sides of the reaction electrode layer 15. Among these parts, the separators 20A, 20B are formed from a gas-impermeable electroconductive member. Plural ribs 22 formed of small projecting pieces are arranged on both surfaces 31 of the separators.
When these separators 20A, 20B are assembled in the fuel cell, the rib (not shown) formed on the surface of the separators 20A at the cathode side constitutes a passage for oxidizing gas supplied to the cathode 12. While the rib 22 formed on the surface 21 of the separator 20B at the anode side constitutes a passage for fuel gas supplied to the anode (not shown). Meanwhile the rib 22 formed on the surface 21 opposite to the above surface of the separator 20A constitutes a passage for fuel gas supplied to the anode (not shown) of another adjacent unit cell (not shown) and a rib (not shown) formed on the surface opposite to the above surface of the separator 20B constitutes a passage for oxidizing gas supplied to a still another adjacent unit cell (not shown). One separator, therefore, supplies both types of gas to adjacent reaction electrodes and prevents mixture of both gases.
Oxidizing gas flowing through the oxidizing gas passage is distributed into the reaction electrode layer exposed to the oxidizing gas passage, and is supplied to the cathode of the reaction electrode layer. Likewise, fuel gas flowing through the fuel gas passage is distributed into the reaction electrode layer exposed to the fuel gas passage, and is supplied to the anode of the reaction electrode layer. As a consequence the respective gas is used in the reaction electrode layer 15 for the electrochemical reaction to produce electromotive force.
Specifically, in the reaction electrode layer 15, the reactions indicated by the formula (1) and the formula (2) proceed at the anode and cathode sides respectively and, on the whole, the reaction indicated by the formula (3) proceeds.H2→2H++2e−  (1)
                                                        1              2                        ⁢                          O              2                                +                      2            ⁢                          H              +                                +                      2            ⁢                          e              -                                      ->                              H            2                    ⁢          O                                    (        2        )                                                      H            2                    +                                    1              2                        ⁢                          O              2                                      ->                              H            2                    ⁢          O                                    (        3        )            
The serpentine type separator has a narrow gas inlet and a long gas passage, resulting in excellent gas diffusibility.
However, in the known serpentine type separator, a partial pressure of gas in the gas passage is not constantly uniform. Accordingly there is the possibility that the performance of the fuel cell as a battery may be deteriorated.
In the lattice type separator, even if one passage is clogged due to, for example, flooding or the like, specifically, condensation of water, gas and produced water can flow into other passages. So this type has excellent drainage as well as high diffusibility of gas. However, in the known lattice type separator, the passages are distributed in forward and backward directions leading to the possibility of insufficient gas flow rate. A deficiency in gas flow rate interrupts diffusion of gas, which causes concentration polarization, resulting in deteriorated performance of the fuel cell as a battery.
In the case of using dry gas at a low humidity as the supply gas (fuel gas and oxygen-containing gas), drainage at the electrode side to which oxygen-containing gas is supplied is excessive. Hence there is the case where an electrolyte film is dried up. This gives rise to the possibility of deteriorating characteristics of the cell.